The speed of any d.c. motor can be altered by a change in any of the variables in the fundamental speed equation: ##EQU1## WHERE S = the speed
k = a constant of proportionality PA1 Va = the armature voltage PA1 Ia = the armature current PA1 Ra = the armature resistance PA1 .phi. = the flux
In many applications such as in the operation of d.c. motors in rolling mills, it is the practice to control the armature voltage from standstill up to base speed, the field flux being held substantially constant, and then increasing the speed beyond base speed by field weakening, i.e. field flux control.
Prior art techniques for accomplishing these objectives are depicted in FIGS. 4A and 4B. In the FIG. 4A a tandem arrangement is utilized using cooperating reference and field rheostats. The reference rheostat has a portion shorted out (as indicated by the cross hatched section) as does a complementary section on the field rheostat. The displacements of the respective wipers are such that when one wiper is contacting a shorted section, the other wiper is actively changing the effective ohmic magnitude in the circuit in which it operates. Thus the reference signal to control the variable power source increases up to some magnitude, and then the field rheostat cuts in additional resistance to decrease the field current and hence the field flux for the motor.
In FIG. 4B separate reference limiter and shaping networks provide motor armature and field flux controls respectively.